In recent years, significant developments in laser technology have led to its application in the field of ophthalmic surgery. In particular, laser surgery has become the technique of choice for ophthalmic surgical applications. In certain ophthalmic laser procedures, surgeons use a mechanical device termed a microkeratome to cut a layer of the anterior surface of the cornea in order to expose the underlying corneal stroma to which the laser is applied. However, complications surrounding the use of the microkeratome with a metal blade have resulted in research into improved techniques that are performed exclusively by a laser system. Such all-laser techniques provide significant improvements over conventional mechanical devices.
Despite these advances in laser technology, the use of such systems for ophthalmic surgical procedures remains fraught with substantial mechanical limitations, particularly in the area of developing a stable interface between an incident laser beam and the eye of a patient. Ophthalmic surgery is a precision operation and requires precise coupling between the surgical tool (i.e., the laser beam) and the region to be disturbed (i.e., a portion of the patient's eye). Movement of the eye with respect to the intended focal point of the laser beam can lead to non-optimal results and might result in permanent damage to non-renewable tissue within the eye. Given that eye movement is often the result of autonomic reflex, techniques have been developed in an attempt to stabilize the position of a patient's eye with respect to an incident laser beam.
One technique used to compensate for relative eye motion with respect to an incident laser beam is to have the patient focus on a stationary target. This involves providing a visual target to the eye undergoing surgery, and requiring that the patient retain focused on the perceived target feature. While this technique has provided some benefit, the patient bears a significant burden of minimizing relative motion. This technique is also less tolerant of any significant gross autonomic reflex motions, e.g., as when the patient might be startled. In this technique, the target provides an optical interface, while the patient's conscious responses provide the feedback mechanism.
Another technique involves the use of an optical eye tracking apparatus, whereby a selected eye feature is targeted for monitoring by an optical device. As the targeted feature displaces as a result of eye movement, this displacement is characterized and fed into the incident laser beam control apparatus as a compensation signal. This technique offers a substantial improvement over the first, particularly when implemented in addition to a patient-driven target focusing mechanism. However, such systems are inordinately expensive since a second, completely independent optical path is typically provided between a patient's eye and a surgical apparatus in order to accommodate the eye tracking apparatus. Further expense and complexity is incurred since an eye tracking apparatus requires an additional software component in order to be operative, which software component must be integrated into a laser delivery system. Considerations of interoperability must be met as well as the provision for an automatic shutdown of the laser system in the event of the loss of target feature lock.
Mechanical stabilization devices have been proposed, for example, a corneal applanation device, which is the subject of U.S. patent application Ser. No. 09/172,819, filed Oct. 15, 1998, and commonly owned by the assignee of embodiments of this invention. Such a mechanical device directly couples a patient's eye to the laser's delivery system being affixed to both the laser and the anterior surface of a patient's cornea. The corneal coupling, in these devices, is typically implemented by lowering an applanation fixture over the anterior surface of the cornea under pressure. It is assumed in these forms of devices that pressure applied normal to the corneal surface will restrict conventional motion of the cornea thereby stabilizing the eye along a major access normal to the device.
However, although this assumption may hold true in a large number of cases, it certainly does not have universal application. Moreover, in the cases where it does hold, the device/cornea interface should be established with the iris centered, for best results. The actual establishment of an effective device/corneal interface is an exercise in trial-and-error, resulting in a great deal of frustration to doctor and patient, as well as considerable eye fatigue.
For ophthalmic laser procedures where eye tissue is to be photodisrupted, it is desirable to have proper focus of the laser beam to a specific focal spot in the tissue that is to be affected. Proper focus includes focal definition and proper dimensionality (i.e., the correct spot diameter and shape). To this end, it is helpful for the laser beam to be as free from aberrations as possible. In particular, for ophthalmic laser procedures involving the cornea, the spherical geometry of the cornea can introduce optical aberrations by its shape, and these are separate and distinct from aberrations that may be introduced by the laser optical system. Corneal induced aberrations can distort the definition of the focal spot of a laser beam as the beam is focused to a position within corneal tissue or deeper into the eye, such as the capsular bag or the natural lens.
Due to the spherical geometry of the anterior surface of the cornea, two specific types of aberrations are of particular importance with regard to beam distortion; spherical aberration (which relates to points on the optical axis of the laser beam) and coma (which relates to points that are off-axis). Spherical aberration and coma are similar to one another in that they both arise from a failure to image or focus optical ray traces onto the same point. Spherical aberration relates to a distortion that can be characterized as radial in nature, with some radial directions being stretched while other radial directions are shrunk, converting thereby an ideally circular spot into an elliptical spot. Coma distortion, on the other hand, implies an elongation along one radius a circle, resulting in a “comet-like” shape. Accordingly, any structure which interfaces between a curved, anterior surface of the cornea and laser delivery system will likely encounter such aberration concerns.
In view of the foregoing, it is thus evident that there is a need for a simple mechanical interface device that is able to stabilize the eye against relative motion with respect to a laser beam used for ophthalmic surgical procedures without relying on secondary mechanical considerations, such as surface tension, friction, or the like. Such a device should be able to present an optical feature to an incident laser beam in a stable, well characterized location. In addition to maintaining a proper orientation between the eye and a laser delivery system during ophthalmic laser surgery, such a device should minimize intraocular pressure during the surgical procedure. Such a device should be easy for a clinician to affix, as well as being simple and cost effective to manufacture and use.